Imaging lens

ABSTRACT

An imaging lens ( 100 ) comprising, arranged sequentially from the object side, a positive-meniscus first lens ( 1 ) with its convex plane facing the object side, a negative-power-meniscus second lens ( 2 ), and a positive-power third lens ( 3 ), the second and third lenses ( 2, 3 ) functioning as correction lenses. The first lens ( 1 ) has a strong power, and both the second and third lenses ( 2, 3 ) are aspherical on opposite planes. When the synthetic focal distance of the imaging lens is f, the focal distance of the first lens f 1 , the distance from the incident surface on the object side to the imaging surface of the first lens ( 1 ) Σd, and the Abbe number of the second lens √d 2 , the following conditional expressions are satisfied. 0.50&lt;f 1 /f&lt;1.5 (1) 0.50&lt;Σd/f&lt;1.5 (2) 50&gt;√d 2  (3). Accordingly, a small, low-cost imaging lens capable of high-quality imaging can be realized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 10/532 382, filed Apr. 21, 2005.

TECHNICAL FIELD

The present invention relates to a small, lightweight imaging lens usedin an automobile-mounted camera, a surveillance camera, a digitalcamera, a camera installed in a mobile telephone, or the like that usesa CCD, CMOS, or other photosensor.

BACKGROUND ART

It is desired that an imaging lens installed in surveillance cameras,digital cameras, and other devices that use CCD, CMOS, or otherphotosensors be provided with the ability to faithfully reproduce thesubject. CCDs themselves or CCD cameras have also been made smaller inrecent years, which has been accompanied by an inevitable increase inthe demand for miniaturization and compact design of the imaging lensesincorporated therein. CCD and other photosensors have also been providedwith high resolution on the order of millions of pixels in contrast withCCD miniaturization. It has inevitably become necessary for the imaginglenses used in cameras having such sensors to also be capable ofdemonstrating high optical performance. In the past, in order todemonstrate high optical performance, aberration had been correctedusing large numbers of lens elements.

A characteristic of a CCD, CMOS, or other photosensor is that the rayangle of each pixel incorporated therein is limited. In a cameraequipped with an optical system that ignores this characteristic, theperipheral light intensity is reduced, and shading occurs. In order tocompensate for these effects, methods have been employed whereby anelectrical correction circuit is provided, or a microlens array thatforms a pair with the photosensor is mounted or the like, and the angleat which light is received on the surface of the element is enlarged orthe like. Alternatively, configurations have been adopted whereby theexit pupil is positioned as far away as possible from the image surface.

On the other hand, there must be a space between the imaging lens andthe CCD in which a low-pass filter, infrared-blocking filter, or thelike is inserted. A limitation therefore exists in that the back focusof the imaging lens must be lengthened to a certain degree.

An imaging lens having high resolution, a small number of lens elements,and a compact structure is disclosed in JP-A 2002-228922. The imaginglens disclosed therein is composed of four elements in three groups, andthe second and third lens groups therein are composed of single lenses.An aspherical surface that contains an inflection point is also employedas the lens surface.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a lightweight, compactimaging lens whereby the maximum exit angle with respect to the elementsurface of the photosensor can be made smaller than the angle of view inorder to prevent shading, and aberration can be corrected in order toadapt to high resolutions of millions of pixels.

Another object of the present invention is to provide a lightweight,compact imaging lens in which an aspherical surface not containing aninflection point is employed in the lens surface, in which aberrationcan be corrected in order to adapt to high resolutions of millions ofpixels, which is advantageous to produce, and which has a small numberof constituent lens elements.

In order to achieve the abovementioned objects, the imaging lensaccording to a first invention of the present application comprisesthree elements in three groups, wherein a first lens with a positivemeniscus whose convex surface faces an object side, a subsequentlypositioned second lens whose meniscus has a negative power, and a thirdlens with positive or negative power are arranged sequentially from theobject side, and the second and third lenses function as correctionlenses. The first lens also has a stronger power compared with thesecond and third lenses. Furthermore, among the first, second, and thirdlenses, both surfaces of at least the second lens and third lens areaspherical. Also, at least one aspherical inflection point is formed atthe aspherical surface of the third lens.

In this arrangement, at least one of the lens surfaces among the lenssurfaces on both sides of the aforementioned first lens can be anaspherical surface.

It is preferred in the imaging lens of the present invention that when atotal focal distance of the imaging lens is f, a focal distance of thefirst lens is f1, a distance from an incident surface of the first lenson the object side to an imaging surface is Σd, and an Abbe number ofthe second lens is vd2, the following conditional expressions aresatisfied.0.50<f1/f<1.5  (1)0.50<Σd/f<1.5  (2)50>vd2  (3)

Conditional expression (1) is a condition for ensuring that sphericalaberration is kept stable and that the lens system as a whole iscompact. If the lower limit is exceeded, the lens system can be madecompact, but it becomes difficult to correct spherical aberration. Ifthe upper limit is exceeded, spherical aberration becomes easy tocorrect, but it becomes impossible to keep the lens system as a wholecompact. By satisfying this conditional expression, the lens system canbe made compact while a satisfactory state of spherical aberration ismaintained.

In the present invention, a total length of the imaging lens can bereduced by making the first lens into a lens with a positive meniscuswhose convex surface faces the object side, and satisfying conditionalexpression (1).

Conditional expression (2) is also a condition for ensuring that thelens system as a whole is more compact. Particularly in the case of animaging lens employed in a camera installed in a mobile telephone, it isnecessary to reduce the size of the lens system as a whole while at thesame time reducing the total length of the lens system. The opticalsystem is preferably designed so as to satisfy conditional expression(2) in order to satisfy these requirements. Below the lower limit ofconditional expression (2), the lens system can be made compact, butvarious types of aberration become difficult to correct. Exceeding theupper limit is also not preferred, because the lens system increases insize.

Conditional expression (3) is a condition for making the Abbe number ofthe second lens equal to 50 or less, and ensuring that the on-axischromatic aberration and the off-axis chromatic aberration are keptstable.

It is also preferred that the third lens in the imaging lens of thepresent invention be configured so that a peripheral portion of a lenssurface thereof on the image side is convex towards the image surface,and that a lens surface thereof on an object side and a lens surfacethereof on the image side are provided with one or a plurality ofaspherical inflection points. By forming the lens surface in thismanner, coma aberration and astigmatic aberration can be satisfactorilycorrected, and distortion can also be satisfactorily corrected.

As a characteristic feature of a case in which the imaging surface is aCCD or CMOS, the ray angle incorporated into each pixel is limited, andthe ray angle increases towards the periphery of the image. It is alsopreferred to mitigate this phenomenon that a configuration be adoptedwhereby the periphery of the lens surface of the third lens on the imageside is an inflected aspherical surface whose convex side faces theimage surface, and that a maximum exit angle of a principal ray is 30degrees or less. Aspherical correction whereby shading is prevented fromoccurring in the periphery of the image is thereby obtained.

An imaging lens according to a second invention of the presentapplication comprises three elements in three groups, wherein a firstlens whose meniscus has a positive power and whose convex surface facesan object side, a second lens whose meniscus has positive or negativepower and whose concave surface faces the object side, and a third lenswith positive power are arranged sequentially from the object side.

Among surfaces of the first, second, and third lenses, the shape of atleast one lens surface is defined by an aspherical shape in which aninflection point does not occur in an effective lens surface regionthereof.

Thus, since the imaging lens of the present invention is a lens systemcomprised of three elements in three groups, and the first lenspositioned on the object side is configured as a lens with a positivemeniscus whose convex surface faces the object side, the total length ofthe lens system can be reduced. By also making the lens surface of thesecond lens on the object side concave, the position of the exit pupilcan be lengthened, whereby shading can be prevented. Furthermore, sincean aspherical shape having no inflection point is employed in the lenssurface, loss of resolution due to lens machining error or assemblyerror can be minimized, and production is facilitated.

In the imaging lens of the present invention herein, when a total focaldistance of the imaging lens is f, a back focus thereof is BF, a focaldistance of the first lens is f1, a curvature of the lens surface of thethird lens on the object side is Ra, and a curvature of the lens surfaceof the third lens on the image side is Rb, it is preferred thatconditional expressions (A) through (C) be satisfied.0.5<f1/f<1.5  (A)0.25<BF/f<1.0  (B)1.0<|Rb/Ra|  (C)

Conditional expression (A) is a condition for ensuring that sphericalaberration is stable and that the lens system as a whole is compact.Below its lower limit, the lens system can be made compact, butspherical aberration becomes difficult to correct. If, conversely, itsupper limit is exceeded, spherical aberration becomes easy to correct,but it becomes impossible to keep the lens system as a whole compact. Bysatisfying this conditional expression, the lens system can be madecompact while a satisfactory state of spherical aberration ismaintained.

In the present invention, the total length of the imaging lens can bereduced by making the first lens into a lens with a positive meniscuswhose convex surface faces the object side, and satisfying conditionalexpression (A).

Conditional expression (B) is also a condition for ensuring that thelens system as a whole is more compact. Particularly in the case of animaging lens employed in a camera installed in a mobile telephone, it isnecessary to reduce the size of the lens system as a whole while at thesame time reducing the total length of the lens system. The opticalsystem is preferably designed so as to satisfy conditional expression(B) in order to satisfy these requirements. Below the lower limit ofconditional expression (B), the lens system can be made compact, but theorganic space between the lens system and the CCD or other imagingsurface is lost, and various types of aberration become difficult tocorrect. Exceeding its upper limit is also not preferred, because thelens system increases in size.

Conditional expression (C) relates to the exit pupil and the back focus,and a condition in which the absolute value of the curvature Ra is equalto or greater than the absolute value of the curvature Rb is notpreferred, because the exit pupil and the back focus are shortened.

Next, when the imaging surface is a CCD, CMOS, or the like, a limit isplaced on the ray angle incorporated into each pixel in orders to ensuresubstantial aperture efficiency. To mitigate this phenomenon, it ispreferred that the exit pupil be lengthened and that the maximum exitangle of the principal ray be corrected to 30 degrees or less. Shadingcan thereby be prevented from occurring in the periphery of the imagesurface. Distortion can also be satisfactorily corrected byappropriately designing the aspherical shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural diagram of the imaging lens according to Example1 in which the first invention of the present application is applied;

FIG. 2 is a structural diagram of the imaging lens according to Example2 in which the first invention of the present application is applied;

FIG. 3 is an aberration diagram of the imaging lens of Example 1 shownin FIG. 1;

FIG. 4 is an aberration diagram of the imaging lens of Example 2 shownin FIG. 2;

FIG. 5 is a structural diagram of the imaging lens according to Examples3 and 5 in which the first invention of the present application isapplied;

FIG. 6 is a structural diagram of the imaging lens according to Example4 in which the first invention of the present application is applied;

FIG. 7 is an aberration diagram of the imaging lens of Example 3 shownin FIG. 5;

FIG. 8 is an aberration diagram of the imaging lens of Example 4 shownin FIG. 6;

FIG. 9 is an aberration diagram of the imaging lens of Example 5 shownin FIG. 5;

FIG. 10 is a structural diagram of the imaging lens of Example A inwhich the second invention of the present application is applied;

FIG. 11 is an aberration diagram of the imaging lens of Example A shownin FIG. 10;

FIG. 12 is a structural diagram of the imaging lens of Examples B and Cin which the second invention of the present application is applied;

FIG. 13 is an aberration diagram of the imaging lens of Example B shownin FIG. 12; and

FIG. 14 is an aberration diagram of the imaging lens of Example C inwhich the second invention of the present application is applied.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of the imaging lens having a three-group, three-elementstructure according to the present invention will be describedhereinafter with reference to the drawings.

EXAMPLE 1

An imaging lens according to Example 1 in which the first invention ofthe present application is applied is depicted in FIG. 1. An imaginglens 100 of the present example has, arranged in sequence from an objectside towards an imaging surface 6, a first lens 1 whose meniscus has apositive power and whose convex surface faces the object side; a secondlens 2 whose meniscus has a negative power and whose concave surfacefaces the object side, positioned subsequently via an aperture 4; and athird lens 3 having a positive power; and the second and third lensesfunction as correction lenses. In the present example, all of the lenssurfaces on both sides of lenses 1, 2, and 3 are aspherical. In thepresent example, a cover glass 5 is mounted between the second lenssurface R6 of the third lens 3 and the imaging surface 6.

In the third lens 3, an aspherical inflection point is provided in thelocation of substantially 50% of the aperture diameter in the first lenssurface R5, and an aspherical inflection point is provided in thevicinity of substantially 25% of the aperture diameter in the secondlens surface R6. The annular zone of the lens periphery of the thirdlens 3 thereby forms a convex surface towards the imaging surface side,and the maximum exit angle of the principal ray is adjusted to 22degrees with respect to the total angle of view of 63 degrees.

The lens data for the entire optical system of the imaging lens 100 ofthe present example are as follows.

-   F-number: 3.5-   Focal distance: f=5.7 mm-   Total length: Σd=7.06 mm

The lens data for the lens surfaces of the imaging lens 100 of thepresent example are shown in Table 1A; and the aspherical coefficientsfor determining the aspherical shape of the lens surfaces are shown inTable 1B. TABLE 1A FNo.: 3.5; f = 5.7 mm; Σd = 7.06 mm i R D Nd νd 1*1.73 1.0 1.5247 56.2 2* 4.46 0.15 3  0.00 0.4 4  0.00 0.5 5* −1.052 0.81.585 29.0 6* −1.50 0.1 7* 5.75 1.2 1.5247 56.2 8* 15.25 1.336 9  0.000.6 1.51633 64.2 10  0.00 0.9779 11 (*indicates an aspherical shape)

TABLE 1B i k A B C D 1   4.740865 × 10⁻²   5.067696 × 10⁻³   4.581707 ×10⁻³ −6.222765 × 10⁻³   3.890559 × 10⁻³ 2   3.767275 × 10⁻¹   3.143529 ×10⁻³ −1.939397 × 10⁻²   9.886734 × 10⁻² −9.132532 × 10⁻² 5 −3.275267 ×10⁻¹   1.603653 × 10⁻²   6.356242 × 10⁻²   2.087871 × 10⁻⁵ −3.891845 ×10⁻² 6 −1.071306 −7.703536 × 10⁻³   1.776501 × 10⁻² 7 2.361313 −1.916465× 10⁻²   6.266366 × 10⁻⁴   5.086988 × 10⁻⁶   6.795863 × 10⁻⁷ 8 0.00−2.213400 × 10⁻²   7.502348 × 10⁻⁴ −3.884072 × 10⁻⁵ −1.070020 × 10⁻⁵

In Table 1A, i indicates the sequence of lens surfaces counted from theobject side; R indicates the curvature of each of the lens surfaces; dindicates the distance between lens surfaces; Nd indicates therefractive index of each of the lenses; and vd indicates the Abbe numberof the lenses. An asterisk (*) by the i of a lens surface indicates thatthe lens surface is aspherical.

When the axis in the optical axis direction is X, the height in thedirection perpendicular to the optical axis is H, the conicalcoefficient is k, and the aspherical coefficients are A, B, C, and D,the aspherical shape employed in the lens surface is indicated by thefollowing equation.$X = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {\left( {k - 1} \right)\left( \frac{H}{R} \right)^{2}}}} + {AH}^{4} + {BH}^{6} + {CH}^{8} + {DH}^{10}}$

The meanings of the symbols and the equations for indicating theaspherical shapes are the same in Examples 2, 3, 4, and 5. In thepresent example, since f1/f=0.84, Σd/f=1.24, and vd2=29, conditionalexpressions (1) through (3) are satisfied.

FIG. 3 is an aberration diagram showing aberrations in the imaging lens100 of Example 1. In the diagram, SA indicates the spherical aberration,OSC indicates the sine condition, AS indicates the astigmaticaberration, and DIST indicates the distortion. The T in the astigmaticaberration AS indicates a tangential image surface, and the S indicatesa sagittal image surface. The aberration diagram at the bottom of thefigure shows the lateral aberration, and in the figure, DX indicates thelaterally directed X aberration relating to the X pupil coordinate; andDY indicates the laterally directed Y aberration relating to the Y pupilcoordinate. The meanings of these symbols are also the same in theaberration diagrams showing the aberration in Examples 2, 3, 4, and 5.

EXAMPLE 2

FIG. 2 is a structural diagram of an imaging lens according to Example 2in which the first invention of the present application is applied. Inan imaging lens 110 of the present example, a first lens 11 with apositive meniscus whose convex surface faces an object side, a secondlens 12 with a negative meniscus whose concave surface faces the objectside, positioned via an aperture 14, and a third lens 13 that is abiconvex lens are arranged in sequence from the object side towards animaging surface 16. An aspherical inflection point is provided in thelocation of substantially 48% of the lens aperture diameter in the firstlens surface R5 of the third lens 13 on the object side. The second lenssurface R6 thereof on the image side is formed as an extension of theconvex surface. Forming the lens surface of the third lens 13 in thismanner allows the maximum exit angle of the principal ray to be 23.5degrees with respect to the total angle of view of 63 degrees. The lenssurfaces of the first lens 11, second lens 12, and third lens 13 of thepresent example are also all aspherical. A cover glass 15 is alsomounted in the present example between the second lens surface R6 of thethird lens 13 and the imaging surface 16.

The lens data for the entire optical system of the imaging lens 110 ofthe present example are as follows.

-   F-number: 3.5-   Focal distance: f=5.7 mm-   Total length: Σd=6.985 mm

The lens data for the lens surfaces of the imaging lens 110 of thepresent example are shown in Table 2A; and the aspherical coefficientsfor determining the aspherical shape of the lens surfaces are shown inTable 2B. In the present example, since f1/f=0.70, Σd/f=1.23, andvd2=29, conditional expressions (1) through (3) are satisfied. Theaberration diagram thereof is shown in FIG. 4. TABLE 2A FNo.: 3.5; f =5.7 mm; Σd = 6.985 mm i R D Nd νd 1* 1.386 1.0 1.5247 56.2 2* 3.087 0.153  0.00 0.18 4  0.00 0.47 5* −0.953 0.9 1.585 29.0 6* −2.016 0.1 7* 6.571.2 1.5247 56.2 8* −6.15 1.336 9  0.00 0.6 1.51633 64.2 10  0.00 1.048911 (*indicates an aspherical shape)

TABLE 2B i k A B C D 1 −2.414289 × 10⁻¹   1.704389 × 10⁻² −7.630913 ×10⁻⁴   1.397945 × 10⁻²   −5.89427 × 10⁻³ 2   7.215993 × 10⁻¹ −3.474378 ×10⁻³ −7.800064 × 10⁻²   9.886734 × 10⁻² −9.132532 × 10⁻² 5   5.484851 ×10⁻¹   1.097456 × 10⁻¹ −2.023164 × 10⁻¹ 5.6317100 × 10⁻¹ −5.506715 ×10⁻¹ 6 −1.456663 −2.197336 × 10⁻² −1.003731 × 10⁻² 7 −3.168123 −1.446476× 10⁻²   1.192514 × 10⁻³   3.793835 × 10⁻⁵ −7.112863 × 10⁻⁶ 8 0.00−1.342604 × 10⁻³ −1.088183 × 10⁻³ −8.566835 × 10⁻⁶   7.766112 × 10⁻⁶

In the imaging lenses 100 and 110 of Examples 1 and 2 above, lenseshaving aspherical surfaces on both sides are used as the first lenses 1and 11 on the object side, but lenses having spherical surfaces on bothsides, or lenses in which at least one of the two surfaces is asphericalmay also be used for the first lenses.

EXAMPLE 3

An imaging lens according to Example 3 in which the first invention ofthe present application is applied is shown in FIG. 5. An imaging lens120 of the present example has, arranged in sequence from an object sidetowards an imaging surface 26, a first lens 21 whose meniscus has apositive power and whose convex surface faces the object side; a secondlens 22 whose meniscus has a negative power and whose concave surfacefaces the object side, positioned subsequently via an aperture 24; and athird lens 23 having a negative power; and the second and third lensesfunction as correction lenses. A cover glass 25 is mounted between thethird lens 23 and the imaging surface 26. In the third lens 23, thesecond lens surface R6 on the side of the imaging surface is formed sothat the annular zone of the lens periphery forms a convex surfacetowards the imaging surface side, and the maximum exit angle of theprincipal ray is adjusted to 24 degrees or less.

In the present example, the both lens surfaces of the first lens 21among the lenses 21, 22, and 23 are spherical. The lens surfaces on theboth sides of the second and third lenses 22 and 23 are aspherical, thesame as in Examples 1 and 2.

The lens data for the entire optical system of the imaging lens 120 ofthe present example are as follows.

-   F-number: 3.5-   Focal distance: f=5.7 mm-   Total length: Σd=6.46 mm

The lens data for the lens surfaces of the imaging lens 120 of thepresent example are shown in Table 3A; and the aspherical coefficientsfor determining the aspherical shape of the lens surfaces are shown inTable 3B. In the present example, since f1/f=0.73, Σd/f=1.13, andvd2=29, conditional expressions (1) through (3) are satisfied. Theaberration diagram thereof is shown in FIG. 7. TABLE 3A FNo.: 3.5; f =5.7 mm; Σd = 6.46 mm i R D Nd νd 1 1.621 1.0 1.5247 56.2 2 5.009 0.15 30.00 0.4 4 0.00 0.5  5* −1.207 0.8 1.585 29.0  6* −1.644 0.1  7* 10.9931.2 1.5247 56.2  8* 7.773 1.336 9 0.00 0.6 1.51633 64.2 10  0.00 0.372611 (*indicates an aspherical shape)

TABLE 3B i k A B C D 5 −2.567837 × 10⁻¹   3.208279 × 10⁻² −1.916911 ×10⁻¹   3.791361 × 10⁻¹ −3.067684 × 10⁻¹ 6 −9.161619 × 10⁻¹ −2.732818 ×10⁻³   1.984030 × 10⁻² 7 6.274432 −2.566783 × 10⁻²   3.344091 × 10⁻³  8.712945 × 10⁻⁵ −2.670618 × 10⁻⁵ 8 0.00 −3.171232 × 10⁻²   1.875582 ×10⁻³ −2.705621 × 10⁻⁴   1.570770 × 10⁻⁵

EXAMPLE 4

FIG. 6 is a structural diagram of an imaging lens according to Example 4in which the first invention of the present application is applied. Inan imaging lens 130 of the present example, a first lens 31 with apositive meniscus whose convex surface faces an object side, a secondlens 32 with a negative meniscus whose concave surface faces the objectside via an aperture 34, and a third lens 33 having a positive power arearranged in sequence from the object side towards an imaging surface 36.A cover glass 35 is mounted between the third lens 33 and the imagingsurface 36. In the third lens 33, the second lens surface R6 is formedso that the annular zone of the lens periphery forms a convex surfacetowards the imaging surface side, and the maximum exit angle of theprincipal ray is adjusted to 24 degrees or less.

In the present example, the both lens surfaces of the first lens 31among the lenses 31, 32, and 33 are spherical. The lens surfaces on theboth sides of the second and third lenses 32 and 33 are aspherical, thesame as in Examples 1, 2 and 3.

The lens data for the entire optical system of the imaging lens 130 ofthe present example are as follows.

-   F-number: 3.5-   Focal distance: f=5.7 mm-   Total length: Σd=6.66 mm

The lens data for the lens surfaces of the imaging lens 130 of thepresent example are shown in Table 4A; and the aspherical coefficientsfor determining the aspherical shape of the lens surfaces are shown inTable 4B. In the present example, since f1/f=0.77, Σd/f=1.17, andvd2=29, conditional expressions (1) through (3) are satisfied. Theaberration diagram thereof is shown in FIG. 8. TABLE 4A FNo.: 3.5; f =5.7 mm; Σd = 6.66 mm i R d Nd νd 1 1.626 1.2 1.4970 81.6 2 4.76 0.15 30.00 0.4 4 0.00 0.5  5* −1.036 0.8 1.585 29.0  6* −1.51 0.1  7* 4.90 1.11.5247 56.2  8* 6.80 0.81 9 0.00 0.6 1.51633 64.2 10  0.00 1.0 11 (*indicates an aspherical shape)

TABLE 4B i k A B C D 5 −6.210503 × 10⁻¹   3.611876 × 10⁻² −2.806078 ×10⁻¹   5.465960 × 10⁻¹ −4.831922 × 10⁻¹ 6 −1.143408   4.811894 × 10⁻³  1.896129 × 10⁻³ 7 1.531998 −2.174083 × 10⁻²   2.450461 × 10⁻³−2.581896 × 10⁻⁴   1.113489 × 10⁻⁵ 8 0.00 −3.318003 × 10⁻²   4.413864 ×10⁻³ −5.477590 × 10⁻⁴   2.739709 × 10⁻⁵

EXAMPLE 5

Referring again to FIG. 5, an imaging lens 140 will be described whereininstead of the first lens 21 in which the both lens surfaces arespherical in the imaging lens 120 of Example 3, a first lens 41 is usedin which one lens surface is formed with an aspherical surface, and theother lens surface is formed with a spherical surface. In FIG. 5,symbols indicating the imaging lens 140 and the first lens 41 areenclosed in parentheses, the configuration of the other parts thereof isthe same as in Example 3, and a description will therefore be givenusing the same symbols.

The imaging lens 140 of the present example has, arranged in sequencefrom an object side towards an imaging surface 26, the first lens 41whose meniscus has a positive power and whose convex surface faces theobject side; a second lens 22 whose meniscus has a negative power andwhose concave surface faces the object side via an aperture 24; and athird lens 23 having a positive power; and the second and third lensesfunction as correction lenses. A cover glass 25 is mounted between thethird lens 23 and the imaging surface 26. In the third lens 23, thesecond lens surface R6 is formed so that the annular zone of the lensperiphery forms a convex surface towards the imaging surface, and themaximum exit angle of the principal ray is adjusted to 24 degrees orless.

In the present example, of the two lens surfaces of the first lens 41among the lenses 41, 22, and 23, the first lens surface R1 on the objectside thereof is aspherical, and the second lens surface R2 on theimaging surface side thereof is spherical. The lens surfaces on the bothsides of the second and third lenses 22 and 23 are aspherical.

The lens data for the entire optical system of the imaging lens 140 ofthe present example are as follows.

-   F-number: 3.5-   Focal distance: f=5.7 mm-   Total length: Σd=7.07 mm

The lens data for the lens surfaces of the imaging lens 140 of thepresent example are shown in Table 5A; and the aspherical coefficientsfor determining the aspherical shape of the lens surfaces are shown inTable 5B. In the present example, since f1/f=0.83, Σd/f=1.24, andvd2=29, conditional expressions (1) through (3) are satisfied. Theaberration diagram thereof is shown in FIG. 9. TABLE 5A FNo.: 3.5; f =5.7 mm; Σd = 7.07 mm i R d Nd νd  1* 1.77 1.0 1.5247 56.2 2 4.973 0.15 30.00 0.4 4 0.00 0.5  5* −1.074 0.8 1.5850 29.0  6* −1.584 0.1  7* 5.5161.2 1.5247 56.2  8* 19.41 1.336 9 0.00 0.6 1.51633 64.2 10  0.00 0.98511 (*indicates an aspherical shape)

TABLE 5B i k A B C D 1 −4.356005 × 10⁻²   8.423055 × 10⁻³ −4.071931 ×10⁻³   4.637228 × 10⁻³ −1.088690 × 10⁻³ 5 −3.998108 × 10⁻¹   3.950244 ×10⁻² −4.246316 × 10⁻²   1.535713 × 10⁻¹ −1.460498 × 10⁻¹ 6 −1.324467−1.748017 × 10⁻³   1.297864 × 10⁻² 7 3.313169 −2.172623 × 10⁻²  1.551952 × 10⁻³ −2.195645 × 10⁻⁵ −1.380375 × 10⁻⁵ 8 0.00 −2.288283 ×10⁻²   1.359618 × 10⁻³ −1.163401 × 10⁻⁴   1.446310 × 10⁻⁶

EXAMPLE A

FIG. 10 is a structural diagram of an imaging lens according to ExampleA in which the second invention of the present application is applied.In an imaging lens 200, a first lens 201 whose meniscus has a positivepower and whose convex surface faces an object side, an aperture 204, asecond lens 202 whose meniscus has a negative power and whose concavesurface faces the object side, and a third lens 203 having a positivepower are arranged in sequence from the object side towards an imagingsurface 206. A cover glass 205 is mounted between the second surface 203b of the third lens 203 and the imaging surface 206.

In this arrangement, the lens surfaces 201 a and 201 b on both sides ofthe first lens 201, the lens surfaces 202 a and 202 b on both sides ofthe second lens 202, and the lens surfaces 203 a and 203 b on both sidesof the third lens 203 are aspherical. All of the aspherical shapesemployed in the present example are also such that no inflection pointsappear in the effective lens surface regions of the lens surfaces.

The lens data for the entire optical system of the imaging lens 200 areas follows.

-   F-number: 2.8-   Focal distance: f=3.65 mm-   Back focus: BF=1.863 mm-   Focal distance of first lens 201: f1=3.769 mm

The lens data for the lens surfaces of the imaging lens 200 are shown inTable 6A, and the aspherical coefficients for determining the asphericalshape of the lens surfaces thereof are shown in Table 6B. TABLE 6A FNo.:2.8; f = 3.65 mm i R d Nd νd 1* 1.153 0.8 1.5247 56.2 2* 2.105 0.15 3 0.00 0.35 4* −1.066 0.7 1.5850 29.0 5* −1.546 0.1 6* 3.180 0.9 1.524756.2 7* 60.657 0.563 8  0.00 0.3 1.51633 64.2 9  0.00 1.0(*indicates an aspherical shape)

TABLE 6B i k A B C D 1   4.577272 × 10⁻¹ −3.645425 × 10⁻³ −2.554281 ×10⁻²   2.607501 × 10⁻² 2 −2.153226   5.788633 × 10⁻² 4.7621418 × 10⁻¹ 4−2.641633 × 10⁻² 5 −6.245341 × 10⁻¹ 6 −1.167034 × 10    1.864785 × 10⁻²−1.905218 × 10⁻³ −6.772919 × 10⁻⁴ 2.049794 × 10⁻⁴ 7 −1.749072 × 10⁵ 

In Tables 6A and 6B, i indicates the sequence of lens surfaces countedfrom the object side; R indicates the curvature of each lens surface onthe optical axis L; d indicates the distance between lens surfaces; Ndindicates the refractive index of each lens; and vd indicates the Abbenumber of the lenses. An asterisk (*) by the i of a lens surfaceindicates that the lens surface is aspherical. The aspherical shapesemployed in the lens surfaces can be indicated by the equation shown inthe description of Example 1.

The meanings of each symbol and the equation for indicating theaspherical shape are also the same in Examples B and C below.

In the present example, the focal distance f1 of the first lens 201 is avalue within the range of 0.5 f (=1.825 mm) and 1.5 f (=5.475 mm), andsatisfies conditional expression (A). The value of BF/f is 0.5109 . . ., and satisfies conditional expression (B). Furthermore, since thecurvature Ra of the lens surface 203 a on the object side of the thirdlens 203 is 3.180, and the curvature Rb of the lens surface 203 b on theimage side thereof is 60.657, then Rb/Ra=19.074 . . . , and conditionalexpression (C) is satisfied. The maximum exit angle of the principal rayis also 30 degrees or less.

FIG. 11 is an aberration diagram showing aberrations in the imaging lensof Example A. FIG. 11(a) is an aberration diagram showing the sphericalaberration SA; FIG. 11(b) is an aberration diagram showing theastigmatic aberration AS; and FIG. 11(c) is an aberration diagramshowing the distortion DIST. The T in the astigmatic aberration ASindicates a tangential image surface, and the S indicates a sagittalimage surface. FIG. 11(d) shows the lateral aberration, and in thefigure, DX indicates the laterally directed X aberration relating to theX pupil coordinate; and DY indicates the laterally directed Y aberrationrelating to the Y pupil coordinate. The meanings of these symbols arealso the same in Examples B and C described hereinafter.

EXAMPLE B

FIG. 12 is a structural diagram of an imaging lens according to ExampleB in which the second invention of the present application is applied.In an imaging lens 210, a first lens 211 whose meniscus has a positivepower and whose convex surface faces an object side, an aperture 214, asecond lens 212 whose meniscus has a positive power and whose concavesurface faces the object side, and a third lens 213 having a positivepower are arranged in sequence from the object side towards an imagingsurface 216. A cover glass 215 is mounted between the third lens 213 andthe imaging surface 216, the same as in Example A. In the case of thepresent example, the lens surfaces 211 a and 211 b on both sides of thefirst lens 211, the lens surfaces 212 a and 212 b on both sides of thesecond lens 212, and the lens surface 213 b on image side of the thirdlens 213 are aspherical. All of the aspherical shapes employed in thepresent example are also such that no inflection points appear in theeffective lens surface regions of the lens surfaces.

The lens data for the entire optical system of the imaging lens of thepresent example are as follows.

-   F-number: 3.5-   Focal distance: f=3.5 mm-   Back focus: BF=1.992 mm-   Focal distance of first lens 211: f1=4.733 mm

The lens data for the lens surfaces of the imaging lens 210 are shown inTable 7A, and the aspherical coefficients for determining the asphericalshape of the lens surfaces thereof are shown in Table 7B. TABLE 7A FNo.:3.5; f = 3.50 mm i R d Nd νd 1* 1.155 0.8 1.5850 29.0 2* 1.475 0.25 3 0.00 0.25 4* −1.234 0.8 1.5247 56.2 5* −1.31 0.15 6  5.87 0.75 1.607029.9 7* −27.245 0.3 8  0.00 0.6 1.51633 64.2 9  0.00 1.092 10  11 (*indicates an aspherical shape)

TABLE 7B i k A B C D 1 6.288194 × 10⁻¹   8.880798 × 10⁻³ −3.552012 ×10⁻²   5.541189 × 10⁻² −2.595815 × 10⁻³ 2 5.605423 −5.846783 × 10⁻²  3.132873 × 10⁻¹ 7.279427 −2.513030 × 10  4 2.369842   1.156048 × 10⁻¹1.324990 5 4.089558 × 10⁻¹   5.253695 × 10⁻²   1.227547 × 10⁻¹ −5.871821× 10⁻²   9.212771 × 10⁻² 7 0.00   −1.935001 × 10⁻²   1.343275 × 10⁻³

In the present example, the focal distance f1 of the first lens 211 is avalue within the range of 0.5 f (=1.75 mm) and 1.5 f (=5.25 mm), andsatisfies conditional expression (A). The value of BF/f is 0.549 . . . ,and satisfies conditional expression (B). Furthermore, since thecurvature Ra of the lens surface 213 a on the object side of the thirdlens 213 is 5.87, and the curvature Rb of the lens surface 213 b on theimage side thereof is −27.245, then |Rb/Ra|=4.641 . . . , andconditional expression (C) is satisfied. The maximum exit angle of theprincipal ray is also 30 degrees or less.

FIGS. 13(a) through 13(d) are aberration diagrams showing theaberrations in the imaging lens 20 of the present example.

EXAMPLE C

The configuration of an imaging lens according to Example C in which thesecond invention of the present application is applied is the same asthe configuration of the imaging lens 210 of Example B, and a first lens211 whose meniscus has a positive power and whose convex surface facesan object side, an aperture 214, a second lens 212 whose meniscus has anegative power and whose concave surface faces the object side, and athird lens 213 having a positive power are arranged in sequence from theobject side toward an imaging surface 216 therein. A cover glass 215 ismounted between the third lens 213 and the imaging surface 216.Furthermore, in the present example, the lens surfaces 211 a and 211 bon both sides of the first lens 211, the lens surfaces 212 a and 212 bon both sides of the second lens 212, and the lens surfaces 213 a and213 b on both sides of the third lens 213 are each aspherical. All ofthe aspherical shapes are also such that no inflection points appear inthe effective lens surface regions of the lens surfaces.

The lens data for the entire optical system of the imaging lens of thepresent example are as follows.

-   F-number: 2.8-   Focal distance: f=3.60 mm-   Back focus: BF=1.967 mm-   Focal distance of first lens 211: f1=3.844 mm

The lens data for the lens surfaces in the imaging lens of the presentexample are shown in Table 8A, and the aspherical coefficients fordetermining the aspherical shape of the lens surfaces thereof are shownin Table 8B. TABLE 8A FNo.: 2.8; f = 3.60 mm i R D Nd νd 1* 1.109 0.851.5247 56.2 2* 1.814 0.25 3  0.00 0.25 4* −0.908 0.7 1.585 29.0 5*−1.638 0.1 6* 3.115 0.95 1.5247 56.2 7* −4.464 0.4 8  0.00 0.3 1.5163364.2 9  0.00 1.267 10  11 (*indicates an aspherical shape)

TABLE 8B i k A B C D 1   3.430395 × 10⁻¹   5.175761 × 10⁻³ 1.822436 ×10⁻³ −3.968977 × 10⁻² 4.390863 × 10⁻² 2 −1.192756 × 10    2.602025 ×10⁻¹ 2.316038 × 10⁻¹ 4   4.565904 × 10⁻¹ 5 −7.957068 × 10⁻¹ −1.046904 ×10⁻¹ 7.812309 × 10⁻³ 6 −2.865732 × 10  −1.057338 × 10⁻² 1.542895 × 10⁻²−6.212535 × 10⁻³ 8.950190 × 10⁻⁴ 7 −2.000000 −7.488042 × 10⁻³

In the present example, the focal distance f1 of the first lens 211 is avalue within the range of 0.5 f (=1.80 mm) and 1.5 f (=5.40 mm), andsatisfies conditional expression (A). The value of BF/f is 0.546 . . . ,and satisfies conditional expression (B). Furthermore, since thecurvature Ra of the lens surface 213a on the object side of the thirdlens 213 is 3.115, and the curvature Rb of the lens surface 213 b on theimage side thereof is −4.464, then |Rb/Ra|=1.433 . . . , and conditionalexpression (C) is satisfied. The maximum exit angle of the principal rayis also 30 degrees or less.

FIGS. 14(a) through 14(d) are aberration diagrams showing theaberrations in the imaging lens of the present example.

(Other Embodiments of the Second Invention)

The lens surfaces on both sides of the first through third lenses areall aspherical in Examples A and C, and the lens surfaces on both sidesof the first lens, on both sides of the second lens, and on the lenssurface of the third lens facing the image side are aspherical inExample B. It is apparent that at least one lens surface among theselens surfaces may be aspherical, and that the other lens surfaces may bespherical.

INDUSTRIAL APPLICABILITY

As described above, the imaging lens according to the first invention ofthe present application is a lens composed of three elements in threegroups, the second lens and third lens are correction lenses, the firstlens positioned on the object side is configured as a positive meniscus,and the convex surface thereof faces the object side. As a result, thetotal length of the lens system can be reduced. Since the lens surfaceof the third lens is configured as an aspherical surface provided withone or a plurality of aspherical inflection points, various types ofaberration can be satisfactorily corrected, while at the same time, themaximum exit angle of the principal ray can be reduced and shadingprevented. Furthermore, aberration can be adequately corrected by thetwo correction lenses that include the second lens and the third lens.Therefore, according to the present invention, a small, compact imaginglens can be obtained that is adapted to high resolution on the order ofmillions of pixels.

Since the imaging lens according to the second invention of the presentapplication is a lens system composed of three elements in three groups,and the first lens positioned on the object side is configured as a lenswith a positive meniscus whose convex surface faces the object side, thetotal length of the lens system can be reduced. By also making the lenssurface of the second lens facing the object side concave, the positionof the exit pupil can be lengthened, whereby shading can be prevented.Furthermore, since an aspherical shape having no inflection points isemployed in the lens surface, loss of resolution due to lens machiningerror or assembly error can be minimized, and production is facilitated.Thus, by the present invention, a small-size, compact imaging lenshaving a small number of constituent lens elements can be obtained thatis suited for production and adapted to high resolution on the order ofmillions of pixels.

1-10. (canceled)
 11. An imaging lens comprising a first lens, a secondlens, and a third lens arranged in sequence from an object side; whereinthe first lens is a meniscus lens having a positive power, whose convexsurface faces the object side; the second lens is a meniscus lens havinga positive or negative power, whose concave surface faces the objectside; the third lens is a lens having a positive power; and a shape ofat least one of lens surfaces of the first, second, and third lenses isdetermined by an aspherical shape in which an inflection point does notappear within an effective lens surface region thereof.
 12. The imaginglens according to claim 11, wherein the following condition is satisfiedwhen a total focal distance of the imaging lens is f and a focaldistance of the first lens is f1:0.5<f1/f<1.5.
 13. The imaging lens according to claim 11, wherein thefollowing condition is satisfied when a total focal distance of theimaging lens is f, and a back focus thereof is BF:0.25<BF/f<1.0
 14. The imaging lens according to claim 11, wherein thefollowing condition is satisfied when a curvature of the lens surface ofthe third lens on the object side is Ra, and a curvature of the lenssurface thereof on the image side is Rb:1.0<|Rb/Ra|.
 15. The imaging lens according to claim 14, wherein amaximum exit angle of a principal ray is 30 degrees or less.
 16. Theimaging lens according to claim 12, wherein the following conditions aresatisfied when a back focus of the imaging lens is BF, a curvature ofthe lens surface on the object side of the third lens is Ra, and acurvature of the lens surface on the image side of the third lens is Rb:0.25<BF/f<1.01.0<|Rb/Ra|.
 17. The imaging lens according to claim 16, wherein amaximum exit angle of a principal ray is 30 degrees or less.